Basics · 8 min read
What Is Radon?
Radon is a naturally occurring radioactive gas formed by decaying uranium in soil and rock. Here is what it is, where it comes from, and why it collects indoors.
Radon is a radioactive gas that forms underground and seeps upward through soil. It has no color, no odor, and no taste, which is why it went unnoticed as a household hazard for most of the twentieth century.
It is not manufactured, imported, or introduced by anything a homeowner does. It is produced continuously by the natural decay of uranium in rock and soil, and every home sits on ground that produces at least some of it.
Where radon comes from
Uranium is present in trace amounts in nearly all soil and rock. As it decays over geologic time, it produces a chain of intermediate elements, one of which is radium. Radium in turn decays into radon.
Radon is the only member of that chain that is a gas, and that single property is what makes it a health issue. Solid decay products stay locked in the rock. Radon migrates through pore spaces, cracks, and fractures, and where the ground meets a building, some of it comes inside.
Why it accumulates indoors
Outdoors, radon disperses into an effectively infinite volume of air and stays at trivial concentrations. Indoors, the same gas enters a bounded volume that exchanges air with the outside only slowly.
Buildings also tend to be slightly negative in pressure relative to the soil beneath them. Warm air rises and escapes at the top of a house, exhaust fans push air out, and clothes dryers and range hoods do the same. Replacement air comes in wherever it can, including through the foundation.
How radon is measured
In the United States, indoor radon is reported in picocuries per liter of air, abbreviated pCi/L. A picocurie is a measure of radioactive decay events, not of mass or volume, which is why the unit looks unfamiliar.
The EPA action level is 4.0 pCi/L. At or above that number, the agency recommends fixing the home. Between 2.0 and 4.0 pCi/L it recommends considering mitigation, because there is no concentration at which radon is considered free of risk.
Radon decay products
Radon itself is chemically inert and largely breathed back out. The health risk comes from what it becomes. As radon decays it produces short-lived solid particles, often called radon progeny or decay products.
These particles attach to dust and are inhaled, where they lodge in lung tissue and continue emitting alpha radiation at close range. That localized radiation damages the cells lining the airways, and over years of exposure that damage can lead to cancer.
Common misconceptions
A few beliefs about radon are widespread and wrong.
- It is not a problem only in basements. Slab-on-grade homes measure high regularly.
- It is not limited to cold-climate states. It has been documented in every state.
- A neighbor's low result says nothing reliable about the house next door.
- New construction is not automatically safe, and tighter envelopes can raise concentrations.
- Air purifiers do not remove radon in any meaningful way.
Key takeaways
- Radon is a naturally occurring radioactive gas from uranium decay in soil and rock.
- It is invisible and odorless, so measurement is the only way to detect it.
- Indoor accumulation is driven by building pressure, not by anything the occupants do wrong.
- The EPA action level is 4.0 pCi/L, and lower is always better.